High temperature filtration control using water based drilling fluid systems comprising water soluble polymers

ABSTRACT

A method and drilling fluid system for providing filtration control during drilling operations using a drilling fluid system comprising water soluble polymer comprising acrylamide monomer, the method comprising: providing a drilling fluid system having effective rheological properties, the drilling fluid system comprising aqueous base consisting essentially of freshwater, a quantity of from about 21 to 58 kg/m 3  (7.5 to about 20 pounds per barrel) plugging agent, and an amount of bridging agent comprising water soluble polymer comprising acrylamide monomer which exhibits a 5% by weight Brookfield viscosity in freshwater of from about 50 cP to about 100 cP, and; contacting a subterranean formation with the drilling fluid system and maintaining effective filtration control at concentrations of about 10,000 ppm or less monovalent salt, as demonstrated in the laboratory by producing a HPHT filtrate of 25 cm 3 /min or less after 30 minutes at about 149° C. (300° F.) after static aging of said drilling fluid system for a period of 16 hours or more at temperatures of about 260° C. (500° F.).

FIELD OF THE INVENTION

The present application relates to a method for providing filtrationcontrol during drilling operations at high temperatures. The presentapplication also relates to water based drilling fluid systems thatachieve high temperature filtration control using water solublepolymers.

BACKGROUND

Filtration control is an important property of a drilling fluid,particularly when drilling through permeable formations where thehydrostatic pressure exceeds the formation pressure. It is important fora drilling fluid to quickly form a filter cake which effectivelyminimizes fluid loss, but which also is thin and erodable enough toallow product to flow into the wellbore during production.

One type of filtration control additive used in water based fluids iswater-soluble polymer, typically comprising acrylamide monomer. Onedisadvantage of water-soluble polymers is that they have limitedtemperature stability. As wells are drilled deeper, higher bottomholetemperatures are encountered.

Filtration control additives are needed which will quickly form a thin,erodable filter cake, and which have high temperature stability forprolonged periods of time.

BRIEF SUMMARY

The present application provides drilling fluid systems comprising watersoluble polymer(s) comprising acrylamide monomer which exhibit effectiverheological properties and high temperature high pressure (HTHP)filtration control at temperatures of 177° C. (350° F.) or more.

The present application provides drilling fluid systems comprising watersoluble polymer comprising acrylamide monomer which exhibit effectiverheological properties and HTHP filtration control at temperatures of260° C. (500° F.) or more.

The present application provides a method for providing filtrationcontrol during drilling operations using a drilling fluid systemcomprising water soluble polymer comprising acrylamide monomer, themethod comprising: providing a drilling fluid system having effectiverheological properties, the drilling fluid system comprising aqueousbase consisting essentially of freshwater, a quantity of from about 21to 58 kg/m³ (7.5 to about 20 pounds per barrel) plugging agent, and anamount of bridging agent comprising water soluble polymer comprisingacrylamide monomer which exhibits a 5% by weight Brookfield viscosity infreshwater of from about 50 cP to about 100 cP, and; contacting asubterranean formation with the drilling fluid system and maintainingeffective filtration control at concentrations of about 10,000 ppm orless monovalent salt, as demonstrated in the laboratory by producing aHPHT filtrate of 25 cm³/min or less after 30 minutes at about 149° C.(300° F.) after static aging of said drilling fluid system for a periodof 16 hours or more at temperatures of about 260° C. (500° F.).

The present application also provides a drilling fluid systemcomprising: an aqueous base consisting essentially of freshwater; aquantity of from about 21 to 58 kg/m³ (7.5 to 20 lb/bbl) plugging agentcomprising sulfonated asphalt and lignite; bridging agent comprising anamount of water soluble polymer having a 5% by weight Brookfieldviscosity in freshwater of from about 50 cP to about 100 cP, the watersoluble polymer comprising first olefinic amide monomer copolymerizedwith monomer selected from the group consisting of sulfonate monomer anda combination of sulfonate monomer and N-vinyl lactam monomer, whereinthe first olefinic amide monomer has the following structure:

-   -   wherein R is a 1-alkenyl group and R¹ and R² independently are        selected from the group consisting of hydrogen and alkyl groups        having from 1 to 4 carbon atoms;    -   the sulfonate monomer has the following structure:    -   wherein R is selected from the group consisting of polymerizable        alkenyl groups having from about 1 to about 4 carbon atoms; R₁        and R₂ are selected from the group consisting of hydrogen and        methyl groups; and, X is a cation; and,    -   the N-vinyl lactam monomer has the following structure:    -   wherein R⁹, R¹⁰, R¹¹, and R¹² independently are selected from        the group consisting of hydrogen, methyl groups, and ethyl        groups.    -   wherein, after 16 hours of static aging of the drilling fluid        system at about 260° C. (500° F.), the drilling fluid system        exhibits a HPHT filtrate of 25 cm³/min or less after 30 minutes        at about 149° C. (300° F.).

In one embodiment, the application provides a drilling fluid systemcomprising: an aqueous base consisting essentially of freshwater;bridging agent comprising from about 0.7 to about 29 kg/m³ (0.25 lb/bblto about 10.0 lb/bbl) water soluble polymer having a 5% by weightBrookfield viscosity in freshwater of from about 50 cP to about 100 cP,the water soluble polymer comprising acrylamide monomer copolymerizedwith monomer selected from the group consisting of sulfonate monomer anda combination of sulfonate monomer and N-vinyl lactam monomer; and, aquantity of from about 21 to 58 kg/m³ (7.5 to 20 lb/bbl) plugging agentcomprising sulfonated asphalt and lignite; wherein, after 16 hours ofstatic aging of the drilling fluid system at about 260° C. (500° F.),the drilling fluid system exhibits a HPHT filtrate of 25 cm³/min or lessafter 30 minutes at about 149° C. (300° F.).

In one embodiment, the application provides a drilling fluid systemcomprising: an aqueous base consisting essentially of freshwater;bridging agent comprising from about 0.7 to about 29 kg/m³ (0.25 lb/bblto about 10.0 lb/bbl) water soluble polymer having a 5% by weightBrookfield viscosity in freshwater of from about 50 cP to about 100 cP,said water soluble polymer comprising monomer of acrylamide, monomer of2-acrylamido-2-methylpropane sulfonic acid or alkali metal saltsthereof, and N-vinyl pyrrolidone monomer; and, from about 21 to 58 kg/m³(7.5 to 20 lb/bbl) plugging agent comprising sulfonated asphalt andlignite; wherein, after 16 hours of static aging of the drilling fluidsystem at about 260° C. (500° F.), the drilling fluid system exhibits aHPHT filtrate of 25 cm³/min or less after 30 minutes at about 149° C.(300° F.).

In another embodiment, the application provides a drilling fluid systemcomprising: an aqueous base consisting essentially of freshwater;bridging agent comprising from about 0.7 to about 29 kg/m³ (0.25 lb/bblto about 10.0 lb/bbl) water soluble polymer having a 5% by weightBrookfield viscosity in freshwater of from about 50 to about 100 cPcomprising monomers of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid or alkali metal salts thereof; and, plugging agentcomprising a quantity of from about 21 to 58 kg/m³ (7.5 to 20 lb/bbl)sulfonated asphalt comprising lignite; wherein, after 16 hours of staticaging of the drilling fluid system at about 260° C. (500° F.), thedrilling fluid system exhibits a HPHT filtrate of 25 cm³/min or lessafter 30 minutes at about 149° C. (300° F.).

DETAILED DESCRIPTION OF EMBODIMENTS

The present application provides a drilling fluid system which quicklyforms a thin, erodable filter cake effective to minimize filtration of adrilling fluid system into the subterranean formation being treated. Thedrilling fluid system is stable for a prolonged period of time evenafter exposure to high temperatures. Effective filtration control isdemonstrated in the laboratory when, after 16 hours of static aging atthe temperature of expected exposure, the drilling fluid system exhibitsa HPHT filtrate of 25 cm³/min or less after 30 minutes at about 149° C.(300° F.). In one embodiment, the drilling fluid system exhibits a HPHTfiltrate of 25 cm³/min or less after 30 minutes at about 177° C. (350°F.) or more.

During drilling operations, the drilling fluid system maintainseffective filtration control even after exposure to a temperature of149° C. (300° F.) for a period of about 16 hours or more. In oneembodiment, the drilling fluid system maintains effective filtrationcontrol even after exposure to a temperature of 177° C. (350° F.) for aperiod of about 16 hours or more. In one embodiment, the drilling fluidsystem maintains effective filtration control even after exposure to atemperature of 260° C. (500° F.) or more for a period of about 16 hoursor more. In yet another embodiment, the drilling fluid system maintainseffective filtration control even after exposure to a temperature of287° C. (550° F.) or more for a period of about 16 hours or more.

The drilling fluid also is effective when exposed to 10,000 ppm or lessmonovalent salt. In one embodiment, the drilling fluid is exposed to9,000 ppm or less monovalent salt.

The Drilling Fluid System

The drilling fluid system comprises an aqueous base, water solublepolymer, and plugging agent. The drilling fluid system also may containother suitable components.

The Aqueous Base

In one embodiment, the aqueous base is freshwater. Although it ispreferred for the aqueous base to be freshwater, the aqueous base maycomprise about 10,000 ppm or less monovalent salt. In anotherembodiment, the aqueous base may comprise about 9,000 ppm or lessmonovalent salt. Typically, the salt content is a result ofcontamination.

Where present, the monovalent salt typically is the type of salt foundin seawater and/or salt flows. Typically, the monovalent salt is sodiumchloride. However, other monovalent salts may be present. Examples ofmonovalent salts that may be present include, but are not necessarilylimited to sodium chloride, potassium chloride, sodium bromide,potassium bromide, sodium formate, potassium formate, cesium formate andmixtures thereof.

The Water Soluble Polymer

The water soluble polymer is thermally stable and comprises repeat unitsof a first olefinic amide monomer and one or more different olefinicmonomers which, in combination, make the water soluble polymer effectiveas a bridging agent. In one embodiment, the first olefinic amide monomeris acrylamide. In one embodiment, the water soluble polymer is acopolymer of acrylamide monomer and sulfonate monomer. In thisembodiment, an advantageous sulfonate monomer is2-acrylamido-2-methylpropane sulfonic acid or alkali metal salts thereof(“AMPS”). Preferred copolymers of acrylamide monomer and AMPS arecommercially available under the name PYRO-TROL®, a trademark of BakerHughes Incorporated.

In another embodiment, the water soluble polymer is a terpolymercomprising acrylamide monomer, sulfonate monomer, and N-vinyl lactammonomer. In an advantageous embodiment, the water soluble polymers areterpolymers comprising (a) acrylamide monomer; (b)2-acrylamido-2-methylpropane sulfonic acid or alkali metal salts thereof(“AMPS”); and (c) olefinic comonomer selected from the group consistingof acrylate monomer and N-vinyl lactam monomer. In one embodiment, (c)is acrylate monomer. In one embodiment, (c) is N-vinyl-2-pyrrolidone(NVP).

Suitable water soluble polymers generally have a 5% by weight Brookfieldviscosity in freshwater of from 50 cP to 100 cP. Generally, watersoluble polymers meeting this viscosity have a number average molecularweight of from about 500,000 to about 2,500,000. In one embodiment, thewater soluble polymer has a number average molecular weight of fromabout 750,000 to about 1,500,000.

The drilling fluid system generally comprises from about 0.7 to about 29kg/m³ (0.25 lb/bbl to about 6.0 lb/bbl) water soluble polymer. In anadvantageous embodiment, the drilling fluid system comprises 2.8 toabout 11.5 kg/m³ (1 to 4 lb/bbl) water soluble polymer.

First Olefinic Monomer

The first olefinic monomer is derived from acrylamide monomer and hasthe following general structure:

wherein R is a 1-alkenyl group and R¹ and R² independently are selectedfrom the group consisting of hydrogen and alkyl groups having from 1 to4 carbon atoms. In one embodiment, R¹ and R² are selected from the groupconsisting of hydrogen and methyl groups.

Sulfonate Monomer

In one embodiment, the third olefinic monomer is sulfonate monomer. Inone embodiment, the sulfonate monomer has the following generalstructure:

wherein R is selected from the group consisting of polymerizable alkenylgroups having from about 1 to about 4 carbon atoms; R¹ and R² areselected from the group consisting of hydrogen and methyl groups; and, Xis a cation. Suitable cations are selected from the group consisting ofhydrogen and alkali metal. An advantageous cation is hydrogen.

A preferred sulfonate is 2-acrylamido-2-methylpropanesulfonic acid(AMPS), which has the following general structure:

The Third Olefinic Monomer

In one embodiment, the third olefinic monomer is acrylate monomer, whichhas the following general structure:

wherein R¹ and R² independently are selected from the group consistingof hydrogen and alkyl groups having from about 1 to about 4 carbonatoms; R is selected from the group consisting of hydrogen, alkyl groupshaving from about 1 to about 4 carbon atoms, and groups that activatethe unsaturated carbon-carbon bond; and X is selected from the groupconsisting of hydrogen and alkali metal. In one embodiment, X isselected from the group consisting of hydrogen and sodium. In oneembodiment, X is hydrogen. Examples of groups that activate theunsaturated carbon-carbon bond include, for example, groups comprisingnitrogen, phosphorus, and sulfur.

N-Vinyl Lactam Monomer

In another embodiment, the third olefinic monomer is selected fromN-vinyl lactam monomers having the following general structure:

wherein R⁹, R¹⁰, R¹¹, and R¹² independently are selected from the groupconsisting of hydrogen, methyl groups, and ethyl groups. In oneembodiment, R⁹, R¹⁰, R¹¹, and R¹² are the same. In one embodiment, theN-vinyl lactam monomer is N-vinyl-2-pyrrolidone (NVP), which iscommercially available from a number of sources.Mole Percent of Monomers

The mole percent of the various monomers will vary depending upon themonomers present and other factors. The embodiments described areillustrative only.

Where the water soluble polymer is a copolymer of acrylamide monomer andsulfonate monomer, the copolymer comprises less than 20 mole % sulfonatemonomer. In another embodiment where the water soluble polymer is acopolymer of acrylamide monomer and sulfonate monomer, the copolymercomprises about 10 mole % or less sulfonate monomer. In an advantageousembodiment, where the water soluble polymer is a copolymer of acrylamidemonomer and sulfonate monomer. In one embodiment, the water solublepolymer is a copolymer of acrylamide monomer and about 10 mole %sulfonate monomer. In one embodiment, the sulfonate monomer is AMPS.

Where the water soluble polymer is a terpolymer of acrylamidemonomer/sulfonate monomer/N-vinyl lactam monomer, the mole percent ofacrylamide monomer is from about 10 mole % to about 20 mole %; the molepercent of sulfonate monomer is from about 50 mole % to about 60 mole %;and, the mole percent of N-vinyl lactam monomer is from about 25 mole %to about 35 mole %. In one embodiment, the mole percent of acrylamidemonomer is from about 15 mole % to about 50 mole %; the mole percent ofsulfonate monomer is from about 37.5 mole % to about 55 mole %; and, themole percent of N-vinyl lactam monomer is from about 12.5 mole % toabout 30 mole %. In one embodiment having the foregoing percentagecomposition, the N-vinyl lactam monomer is N-vinyl pyrrolidone (NVP).

In one embodiment, the water soluble polymer is a terpolymer having amole ratio of 55:30:15 of AMPS:NVP:acrylamide. In another embodiment,the water soluble polymer is a terpolymer having a mole ratio of50:37.5:12.5 of acrylamide:AMPS:acrylate.

The water soluble polymers may be made by any suitable method include,for example, the bead method and the spray dried powder method.

The Plugging Agent

In order to assist in forming an effective seal for filtration control,one embodiment of the drilling fluid system further comprises a pluggingagent. The plugging agent is sufficiently soluble in the drilling fluidsystem to deform and to penetrate microfractures in the formation, whichassists in sealing the formation. The plugging agent assists inmaintaining filtration control in the presence of contamination bymonovalent salt.

Examples of suitable plugging agents include, but are not necessarilylimited to sized sulfonated asphalt, limestone, marble, mica, graphite,cellulosics, lignins, cellophanes, and latexes. In one embodiment, theplugging agent comprises sulfonated asphalt.

Sulfonated asphalts are generally produced by reacting an asphalticmaterial which is first mixed with a solvent followed by sulfonating theasphaltic material with a sulfonating agent. Suitable solvents includealkanes. An example is hexane. Suitable sulfonating agents includesulfur trioxide. The sulfonation process produces asphalt-sulfonic acid,which is neutralized with a basic compound. Suitable basic compoundsinclude, for example, sodium hydroxide. See U.S. Pat. No. 4,741,868,which is incorporated herein by reference. In one embodiment, thesulfonated asphalt is in the form of a salt. In one embodiment, thesulfonated asphalt is in the form of a sodium salt or a potassium salt.

In an advantageous embodiment, the plugging agent comprises acombination of sulfonated asphalt and lignite. In one embodiment, thesulfonated asphalt comprises about 20 wt. % or more lignite. Suitably,the sulfonated asphalt comprises about 30 wt. % or less lignite. In aparticularly advantageous embodiment, the sulfonated asphalt comprisesfrom about 25 wt. % to about 28 wt. % lignite. A suitable commercialsulfonated asphalt is SULFA-TROLL, which is commercially available fromBaker Hughes Drilling Fluids.

In one embodiment, the drilling fluid system advantageously comprisesfrom about 21 to 58 kg/m³ (7.5 to 20 lb/bbl) of plugging agent. In oneembodiment, the drilling fluid system comprises about 10 lb/bblsulfonated asphalt comprising from about 20 wt. % to about 30 wt. %lignite, based on the total weight of the plugging agent.

In one embodiment, the fluid further comprises thinner. Any suitablethinner may be used. In one embodiment, the thinner comprisescausticized or uncausticized lignite. An exemplary commerciallyavailable lignite is LIGCO®, a trademark of Milwhite Mud Sales Company,Houston, Tex. An exemplary commercially available causticized lignite isLIGCON®, a trademark of Milchem Incorporated, Houston, Tex.

Other additives may be used in the drilling fluid system. Such additivesinclude, but are not necessarily limited to shale stabilizer(s), otherfiltration control additive(s), suspending agent(s), dispersant(s),anti-balling additive(s), lubricant(s), weighting agent(s), seepagecontrol additive(s), lost circulation additive(s), drilling enhancer(s),penetration rate enhancer(s), corrosion inhibitor(s), acid(s), base(s),buffer(s), scavenger(s), gelling agent(s), cross-linker(s), catalyst(s),soluble salts, biocides; one or more additional bridging agent, andcombinations thereof.

The drilling fluid system may be used to drill substantially anyformation. However, not all types of drilling fluid systems encounterextremely high temperatures. In one embodiment, the drilling fluidsystem is a drill-in or completion fluid—fluids which are more likely tobe exposed to higher downhole temperatures for prolonged periods oftime.

The drilling fluid system suitably has a density of about 9 lb/gal ormore. Suitably, the drilling fluid system has a density of about 22lb/gal or less.

The application will be better understood with reference to thefollowing Examples which are for purposes of illustration only:

Procedures

Fluids were prepared using a variety of polymers from a variety ofsources. The fluids were subjected to standard testing to determinerheological and fluid loss properties. HPHT was measured following theAPI test method described in “Recommended Practice Standard Procedurefor Field Testing Water-Based Drilling Fluids,” 113B-1, 3^(rd) edition,incorporated herein by reference. In some instances, the method wasmodified as indicated in the Examples.

In the following formulations, the Experimental Water Soluble Polymersfrom various sources and batches had the following monomeric content:

-   -   A. 10 mole % AMPS polymerized with acrylamide;    -   B. 20 mole % AMPS/acrylamide;    -   C. 40 mole % AMPS:acrylamde;    -   D. 50:37.5:12.5 mole % acrylamide:AMPS: acrylate terpolymer;    -   E. 55:30:15 mole % terpolymer of AMPS:NVP:acrylamide; and,    -   F. 90 weight percent NaAMPS (AMPS, sodium salt)/10 weight        percent N,N-dimethylacrylamide        A number of commercial products are used in the formulations        described in the examples. The following is a description of        these products and their sources:        Products Available from Baker Hushes Drilling Fluids:    -   MIL-GEL™ is a ground montmorillonite used to simulate formation        shale contaminant;    -   MILGEL-NT™ is a ground montmorillonite used to simulate        formation shale contaminant;    -   CHEMTROL®-X is a blend of ground lignitic earth and synthetic        maleic anhydride copolymers;    -   MIL-TEMP® is a maleic anhydride copolymer useful to prevent        thermal degradation of a drilling fluid system;    -   MIL-CARB® and MIL-CARB® 5 are products comprising ground marble        or calcium carbonate particles;    -   MIL-BAR® is a barite additive;    -   DFE-129™ is an acrylamide/AMPS copolymer;    -   CHEK-LOSS® PLUS is a water-insoluble, ultra-fine, (lignin)        material used for controlling seepage and loss of circulation        while drilling through depleted or under-pressured zones;    -   SULFA-TROL® is a sulfonated asphalt;    -   MAX-TROL® is a sulfonated resin additive for water-based mud        systems;    -   KEM-SEAL® PLUS is a water soluble polymer having about 90 wt. %        NaAMPS (AMPS, sodium salt) and 10 wt. % N,N-dimethylacrylamide;    -   ALL-TEMP® is an acrylate tetrapolymer.    -   LIGCON® is causticized lignite;    -   LIGCO® is lignite;    -   PROTECTO-MAGIC™ is a ground asphalt.        Products Available from Other Commercial Sources:    -   REV-DUST is a simulated drilled product which may be obtained        from Mil-White Company, Houston, Tex.;    -   POLYDRILL® is an anionic synthetic polymer commercially        available from Degussa Construction Polymers GMBH Corporation,        Fed. Rep. Germany;    -   BORE-DRILL™ is an anionic synthetic polymer commercially        available from Borden Chemicals;    -   SOLTEX® is a sulfonated asphalt, commercially available from        Chevron Phillips Chemical Company;    -   DRISCAL D is a water soluble polymer provided by Drilling        Specialties Company.        In a number of the examples, POLYDRILL® or another component is        listed twice in order to show a difference in order of addition        while mixing

The application will be better understood with reference to thefollowing examples, which are illustrative only:

EXAMPLE 1

In the following Example, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was 10 mole % AMPSpolymerized with acrylamide. The formula and the results are given inthe following Table: Experimental Water Soluble Polymer A A AFreshwater, ml (lb) 239 239 239 (0.68) (0.68) (0.68) MILGEL ™ NT, kg(lb) 4.5 4.5 4.5 (10.0) (10.0) (10.0) CHEMTROL ® X, kg (lb) 4.5 4.5 4.5(10.0) (10.0) (10.0) Caustic Soda, kg (lb) 0.23 0.23 0.23 (0.5) (0.5)(0.5) MIL-TEMP ®, kg (lb) 0.9 0.9 0.9 (2.0) (2.0) (2.0) 10 mole % AMPSpolymerized 0.9 0.9 0.9 with acrylamide, kg (lb) (2.0) (2.0) (2.0)MIL-CARB ®, kg (lb) 4.5 4.5 4.5 (10.0) (10.0) (10.0) MIL-CARB ® 5, kg(lb) 2.3 2.3 2.3 (5.0) (5.0) (5.0) MIL-BAR ®, kg (lb) 182.7 182.7 182.7(403.0) (403.0) (403.0) FLUID PROPERTIES Fann 35 at 49° C. (120° F.)232° C. 232° C. 232° C. (450° F.) (450° F.) (450° F.) Static Age StaticAge Static Age Initial 16 Hours Initial 16 Hours Initial 16 Hours 600rpm 90 147 72 142 69 140 300 rpm 50 87 39 83 38 82 200 rpm 36 64 28 6028 61 100 rpm 22 39 12 37 12 38  6 rpm 5 9 5 8 5 9  3 rpm 4 7 4 6 4 7Plastic Viscosity, cP 40 60 33 59 31 58 Yield Point, Pa 4.8 12.9 2.911.5 3.4 11.5 (lb/100 ft²) (10) (27) (6) (24) (7) (24) Gels, Pa 2.9/3.44.3/16.3 2.4/3.4 3.8/9.58 2.4/3.4 4.3/11 (lb/100 ft²) (6/7) (9/34) (5/7)(8/20) (5/7) (9/23) API fluid loss, cm³/30 min 2.0 2.3 2.1 2.5 2.4 3.0300° F. HPHT filtrate, — 22 — 19 — 21 cm³/30 min pH 9.1 8.8 9.3 9.0 8.78.7Without added NaCl, after 16 hours of static aging at about 232° C.(450° F.), all of the formulations measured exhibited an HPHT filtrateof 25 cm³/min or less after 30 minutes at about 148.9° C. (300° F.).

EXAMPLE 2

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (D) 50:37.5:12.5mole % acrylamide:AMPS:acrylate or (E) and 55:30:15 weight % terpolymerof AMPS:NVP:acrylamide. The formulas and the results are given in thefollowing Table: Form. 1 2 3 4 5 6 Experimental Water Soluble Polymer DE D E D E Fresh- 0.07 0.07 0.07 0.07 0.07 0.07 water, m³ (0.56) (0.56)(0.56) (0.56) (0.56) (0.56) (bbl) MIL- 2.3 2.3 2.3 2.3 1.8 1.8 GEL ™,(5.0) (5.0) (5.0) (5.0) (4.0) (4.0) kg (lb) MIL- 0.7 0.7 0.7 0.7 1.1 1.1TEMP ®, (1.5) (1.5) (1.5) (1.5) (2.5) (2.5) kg (lb) CHEM- 2.7 2.7 2.72.7 2.3 2.3 TROL ® X, (6.0) (6.0) (6.0) (6.0) (5.0) (5.0) kg (lb)Caustic 0.2 0.2 0.2 0.2 0.2 0.2 soda, (0.5) (0.5) (0.5) (0.5) (0.5)(0.5) kg (lb) KEM- 0.5 0.5 0.5 0.5 0.5 0.5 SEAL ® (1.0) (1.0) (1.0)(1.0) (0.5) (0.5) PLUS, kg (lb) Experimental 0.9 0.9 0.9 0.9 0.9 0.9water (2.0) (2.0) (2.0) (2.0) (2.0) (2.0) soluble polymer, kg (lb) POLY-3.6 3.6 3.6 3.6 3.2 3.2 DRILL ™, (8.0) (8.0) (8.0) (8.0) (7.0) (7.0) lb(kg) MIL- 4.5 4.5 4.5 4.5 4.5 4.5 CARB ®, (10) (10) (10) (10) (10) (10)kg (lb) MIL- 248.4 248.4 248.4 248.4 248.4 248.4 BAR ®, (547.7) (547.7)(547.7) (547.7) (547.7) (547.7) kg (lb) Form. 1 2 3 4 5 6 FLUIDPROPERTIES Fann 35 at 49° C. (120° F.) 204° C. 204° C. 260° C. 260° C.260° C. 260° C. (400° F.) (400° F.) (500° F.) (500° F.) (500° F.) (500°F.) Static Static Static Static Static Static Age Age Age Age Age Age 1616 16 16 16 16 Initial Hours Initial Hours Initial Hours Initial HoursInitial Hours Initial Hours 600 rpm 124 119 188 153 131 >300 108 100 161237 300 rpm 68 82 107 104 108 >300 57 69 92 196 200 rpm 49 69 77 87100 >300 40 60 66 185 100 rpm 29 52 45 65 88 >300 23 48 37 161  6 rpm 629 7 32 71 265 5 30 5 125  3 rpm 4 29 5 32 70 240 4 27 3 122 Plastic 5637 81 49 23 NM* 51 31 69 41 Viscosity, cP Yield 5.7 23.5 12.5 26.3 40.7NM* 2.9 18.2 6.2 72.3 Point, Pa (12) (45) (26) (55) (85) (6) (38) (23)(155) (lb/100 ft²) Gels, Pa 2.4/4.3 17.2/28.7 2.9/5.2 21.1/35.9 39.7/5696.7/98.2 2.4/3.3 16.3/32.6 2.4/4.3 59.4/60 (lb/100 ft²) (5/9) (36/60)(6/11) (44/75) (83/117) (202/205) (5/8) (34/68) (5/9) (124/125) APIfluid 1.4 1.9 1.8 1.8 2.6 — 1.8 3.0 1.3 4.6 loss, cm³/30 min 177° C. —6.0 — 11 <1† — — 16 — — (350° F.) HPHT filtrate, cm³/30 min pH 9.5 8.89.5 8.7 8.4 — 9.5 8.4 9.6 8.5†Possible plugged valve stem during HPHT filtration*Non measurable.Without added NaCl, after 16 hours of static aging at about 204° C.(400° F.) for Formulas 1 and 2 and at 260° C. (500° F.) for Formulas3-6, all of the formulations for which HPHT filtrate was measuredexhibited an HPHT filtrate of 25 cm³/min or less after 30 minutes atabout 177° C. (350° F.). The yield points were more variable thandesired, so further study was undertaken to determine how to stabilizethe yield point, among other things.

EXAMPLE 3

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (D) 50:37.5:12.5mole % acrylamide:AMPS:acrylate. The formulas and the results are givenin the following Table: Form. 7 8 9 10 11 12 Experimental Water SolublePolymer D D D D D D Fresh- 196 196 196 196 196 196 water, (0.56) (0.56)(0.56) (0.56) (0.56) (0.56) ml(bbl) MIL- 1.4 1.6 1.4 2.3 1.4 1.4 GEL ™,(3.0) (3.5) (3.0) (4.0) (3.0) (3.0) kg (lb) MIL- 0.9 0.9 1.1 1.1 0.9 0.9TEMP ®, (2.0) (2.0) (2.5) (2.5) (2.0) (2.0) kg (lb) CHEM- 2.3 2.3 1.12.3 2.3 2.3 TROL ® X, (5.0) (5.0) (2.5) (5.0) (5.0) (5.0) kg (lb)Caustic 0.2 0.2 0.2 0.2 0.2 0.2 soda, (0.5) (0.5) (0.5) (0.5) (0.5)(0.5) kg (lb) KEM- — 0.2 0.2 — — 0.2 SEAL ® (0.5) (0.5) (0.5) PLUS, kg(lb) Experimental 1.1 1.1 1.1 0.9 1.4 0.9 Water (2.5) (2.5) (2.5) (2.0)(3.0) (2.0) soluble polymer, kg (lb) POLY- 3 3 3 3 2.3 1.6 DRILL ™,(7.0) (7.0) (7.0) (7.0) (5.0) (3.5) kg (lb) MIL- 2.3 2.3 2.3 4.5 4.5 4.5CARB ®, (5.0) (5.0) (5.0) (10) (10) (10) kg (lb) MIL- 248.4 248.4 248.4248.4 248.4 248.4 BAR ®, (547.7) (547.7) (547.7) (547.7) (547.7) (547.7)kg (lb) Form. 7 8 9 10 11 12 FLUID PROPERTIES Fann 35 at 49° C. (120°F.) 260° C.) 260° C. 260° C. 260° C. 260° C. 260° C. 500° F.) (500° F.)(500° F.) (500° F.) (500° F.) (500° F.) Static Static Static StaticStatic Static Age 16 Age 16 Age 16 Age 16 Age 16 Age 16 Initial HoursInitial Hours Initial Hours Initial Hours Initial Hours Initial Hours600 rpm 76 85 103 80 104 69 78 96 89 70 118 81 300 rpm 39 52 53 49 54 4439 59 45 44 60 41 200 rpm 27 41 38 37 38 34 27 47 33 33 42 26 100 rpm 1529 21 26 21 24 15 34 19 22 24 13  6 rpm 3 14 4 13 4 11 3 20 4 9 4 2  3rpm 2 12 3 13 3 10 2 19 3 9 3 1 Plastic 37 33 50 31 50 25 39 37 34 26 5840 Viscosity, cP Yield 0.96 9.1 1.4 8.6 1.9 9.1 0 10.5 5.3 8.6 8.6 0.48Point, (2) (19) (3) (18) (4) (19) (0) (22) (11) (18) (18) (1) Pa (lb/100ft²) Gels, 1.4/2.9 8.6/18.2 2.4/2.9 10/19.6 2.4/3.4 8.1/14.8 1.9/2.913.9/33 1.9/3.4 7.7/18.7 2.5/3.4 .96/7.7 Pa(lb/100 ft²) (3/6) (18/38)(5/6) (21/41) (5/7) (17/31) (4/6) (29/69) (4/7) (16/39) (5/7) (2/16) APIfluid 2.5 3.0 1.6 2.6 2.7 4.0 1.9 2.6 3.0 3.2 2.5 2.6 loss, cm³/30 min177° C. — *80 — *50 — — — — — 50 — 50 (350° F.) (6 min) (26 min) HPHTfiltrate, cm³/30 min pH 9.7 8.2 9.5 8.3 9.5 8.3 9.5 8.6 9.5 8.6 9.6 8.5Surprisingly, even in the absence of added NaCl, after 16 hours ofstatic aging at about 260° C. (500° F.), all of the formulations forwhich HPHT filtrate was measured exhibited an HPHT filtrate of greaterthan 25 cm³/min after 30 minutes or less at about 177° C. (350° F.). Theresults are believed to be attributable to variability in the propertiesof the commercially available polymer.

EXAMPLE 4

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (D) 50:37.5:12.5mole % acrylamide:AMPS:acrylate or (A) 10 mole % AMPS polymerized withacrylamide. The formulas and the results are given in the followingTable: Formulation 13 14 15 16 17 Experimental Water Soluble Polymer D DA A D Freshwater, 196 196 176 196 196 ml(bbl) (0.56) (0.56) (0.50)(0.50) (0.56) MIL-GEL ™, 1.4 1.4 2.3 2.3 2.3 kg (lb) (3.0) (3.0) (4.0)(4.0) (4.0) MIL-TEMP ®, 0.9 0.9 1.1 1.1 1.1 kg (lb) (2.0) (2.0) (2.5)(2.5) (2.5) CHEM-TROL ® 2.3 2.3 2.3 2.3 1.1 X, kg (lb) (5.0) (5.0) (5.0)(5.0) (2.5) Caustic soda, 0.23 0.23 0.23 0.23 — kg (lb) (0.5) (0.5)(0.5) (0.5) KEM-SEAL ® 0.5 0.5 0.23 0.23 0.23 PLUS, kg (lb) (1.0) (1.0)(0.5) (0.5) (0.5) Experimental 0.9 0.9 0.9 0.9 0.9 Water soluble (2.0)(2.0) (2.0) (2.0) (2.0) polymer, kg (lb) MAX-TROL ®, 1.6 — — — — kg (lb)(3.5) POLYDRILL ™, — — 3.2 3.2 4 kg (lb) (7.0) (7.0) (9.0) BOREDRILL ™,— 1.6 — — — kg (lb) (3.5) MIL-CARB ®, 4.5 4.5 4.5 4.5 4.5 kg (lb) (10)(10) (10) (10) (10) MIL-BAR ®, 248.4 248.4 248.4 248.4 248.4 kg (lb)(547.7) (547.7) (547.7) (547.7) (547.7) Caustic soda, — — — — Adj. pH to9.0 kg (lb) Formulation 13 14 15 16 17 FLUID PROPERTIES Fann 35 at 49°C. (120° F.) 260° C. 260° C. 260° C. 260° C. 260° C. (500° F.) (500° F.)(500° F.) (500° F.) (500° F.) Static Static Static Static Static Age 16Age 16 Age 16 Age 16 Age 16 Initial Hours Initial Hours Initial HoursInitial Hours Initial Hours 600 rpm 117 71 166 164 233 85 279 123 116 72300 rpm 58 35 89 96 127 47 156 72 66 47 200 rpm 40 24 62 72 91 35 117 5447 37 100 rpm 22 14 34 45 53 23 68 35 27 27  6 rpm 4 2 5 8 9 8 12 10 513  3 rpm 2 2 3 6 7 6 9 8 3 11 Plastic Viscosity, 59 36 77 68 106 38 12351 50 25 Cp Yield Point, −0.47 −0.47 5.7 13.4 10 4.3 15.8 10 7.7 10.5Pa(lb/100 ft²) (−1) (−1) (12) (28) (21) (9) (33) (21) (16) (22) Gels, Pa1.9/2. 2.4/8.1 2.4/3 7.7/23 4.8/9.6 6.2/17.7 6.2/12 7.7/19. 2.4/8.68.6/21 (lb/100 ft²) (4/5) (5/17) (5/6) (16/48) (10/20) (13/37) (13/26)(16/40) (5/18) (18/45) API fluid loss, 2.4 8.2 2.8 >10 2.6 2.2 2.6 2.61.8 3.5 cm³/30 min 177° C. (350° F.) — — — — — NC — >50 — 35 HPHTfiltrate, cm³/30 min pH 9.9 8.3 9.9 8.1 9.0 8.2 9.0 8.3 9.0 8.2Surprisingly, even in the absence of added NaCl, after 16 hours ofstatic aging at about 260° C. (500° F.), all of the formulations forwhich HPHT filtrate was measured exhibited an HPHT filtrate of greaterthan 25 cm³/min after 30 minutes or less at about 177° C. (350° F.). Theresults are believed to be attributable to variability in the propertiesof the commercially available polymer.

EXAMPLE 5

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (D) 50:37.5:12.5mole % acrylamide:AMPS:acrylate or (A) 10 mole % AMPS polymerized withacrylamide. The formulas and the results are given in the followingTable: Formulation 18 19 20 21 22 Experimental water soluble polymer D DA A A Freshwater, ml 196 196 196 196 196 (bbl) (0.56) (0.56) (0.56)(0.56) (0.56) MIL-GEL ™, 2.3 2.3 2.3 2.3 2.3 kg (lb) (4.0) (4.0) (5.0)(5.0) (5.0) MIL-TEMP ®, kg 1.1 1.1 0.7 0.7 0.7 (lb) (2.5) (2.5) (1.5)(1.5) (1.5) CHEM-TROL ® X, 3.2 1.6 2.7 2.7 2.7 kg (lb) (7.0) (3.5) (6.0)(6.0) (6.0) Caustic soda, — — 0.23 0.23 0.23 kg (lb) (0.5) (0.5) (0.5)KEM- 0.5 0.5 0.5 0.5 0.5 SEAL ® PLUS, (1.0) (1.0) (1.0) (1.0) (1.0) kg(lb) Experimental 1.1 1.1 0.9 0.9 0.9 Water Soluble (2.5) (2.5) (2.0)(2.0) (2.0) Polymer, kg (lb) MAX-TROL ®, — — — — — kg (lb) POLYDRILL ™,1.6 3.2 3.6 3.6 3.6 kg (lb) (3.5) (7.0) (8.0) (8.0) (8.0) BOREDRILL ™, —— — — — kg (lb) MIL-CARB ®, 4.5 4.5 4.5 4.5 4.5 kg (lb) (10) (10) (10)(10) (10) MIL-BAR ®, 248.4 248.4 248.4 248.4 248.4 kg (lb) (547.7)(547.7) (547.7) (547.7) (547.7) Caustic soda, Adj. pH to 9.0 Adj. pH to9.0 — — — kg (lb) Formulation 18 19 20 21 22 FLUID PROPERTIES Fann 35 at49° C. (120° F.) 260° C. 260° C. 260° C. 260° C. 260° C. (500° F.) (500°F.) (500° F.) (500° F.) (500° F.) Static Static Static Static Static Age16 Age 16 Age 16 Age 16 Age 16 Initial Hours Initial Hours Initial HoursInitial Hours Initial Hours 600 rpm 168 66 223 112 224 131 300 rpm 92 38123 67 131 88 200 rpm 66 26 87 52 93 73 100 rpm 38 16 50 37 54 55  6 rpm7 3 9 15 9 32  3 rpm 4 2 6 15 7 32 Plastic Viscosity, 76 28 101 45 93 43Cp Yield Point, 7.7 4.8 10 10.5 18.2 21.5 Pa(lb/100 ft²⁾ (16) (10) (21)(22) (38) (45) Gels, 2.9/4.3 3.8/11.5 3.8/6.2_(—) 12/30.6 4.3/7.221.5/39 Pa (lb/100 ft²) (6/9) (8/24) (8/13) (25/64) (9/15) (45/82) APIfluid loss, 2.5 3.4 0.9 1.8 1.7 2.2 cm³/30 min 177° C. (350° F.) — 76 —22 — 14 HPHT filtrate, cm³/30 min pH 9.0 8.2 9.8 9.0 9.8 8.5After 16 hours of static aging at about 260° C. (500° F.), Formula 19exhibited an HPHT filtrate of greater than 25 cm³/min after 30 minutesor less at about 177° C. (350° F.), and Formulas 21 and 22 exhibited anHPHT filtrate of less than 25 cm³/min after 30 minutes at about 177° C.(350° F.). The negative results for Formula 19 are believed to beattributable to variability in the properties of the commerciallyavailable polymer.

EXAMPLE 6

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Form. 21 (con't) 22 (con't) 23 24 ExperimentalWater Soluble Polymer A A A A Freshwater, 196 196 196 176 ml(bbl) (0.56)(0.56) (0.56) (0.50) MIL-GEL ™, 2.3 2.3 2.3 2.3 kg (lb) (5.0) (5.0)(5.0) (5.0) MIL-TEMP ®, 0.7 0.7 0.7 0.7 kg (lb) (1.5) (1.5) (1.5) (1.5)CHEM- 2.7 2.7 2.7 2.7 TROL ® X, (6.0) (6.0) (6.0) (6.0) kg (lb) Causticsoda, 0.23 0.23 0.23 0.23 kg (lb) (0.5) (0.5) (0.5) (0.5) KEM- 0.5 0.50.5 0.5 SEAL ® PLUS, (1.0) (1.0) (1.0) (1.0) kg (lb) Experimental 0.90.9 0.9 0.9 Water (2.0) (2.0) (2.0) (2.0) Soluble Polymer, kg (lb) POLY-3.6 3.6 3.6 3.6 (8.0) (8.0) (8.0) (8.0) DRILL ™, kg (lb) MIL-CARB ®, 4.54.5 4.5 4.5 kg (lb) (10) (10) (10) (10) MIL-BAR ®, 248.4 248.4 248.4248.4 kg (lb) (547.7) (547.7) (547.7) (547.7) Caustic soda, — — — — kg(lb) Form. 21 (con't) 22 (con't) 23 24 FLUID PROPERTIES Fann 35 at 49°C. (120° F.) 260° C. 260° C. 260° C. 260° C. 260° C. 260° C. 260° C.260° C. (500° F.) (500° F.) (500° F.) (500° F.) (500° F.) (500° F.)(500° F.) (500° F.) Static Static Static Static Static Static StaticStatic Age 48 Age 72 Age 48 Age 72 Age 16 Age 48 Age 72 Age 16 HoursHours Hours Hours Initial Hours Hours Hours Initial Hours 600 rpm 113103 169 105 243 124 109 100 >300 96 300 rpm 62 54 111 64 134 79 63 50226 76 200 rpm 45 39 90 49 96 65 47 35 169 68 100 rpm 28 23 67 34 55 4830 21 115 55  6 rpm 9 5 38 14 8 26 9 4 25 40  3 rpm 9 4 38 14 6 27 7 321 40 Plastic 51 49 58 41 109 45 46 50 NM 20 Viscosity, Cp Yield Point,5.2 2.4 25.4 11 12 16.3 8.1 0 NM 26.8 Pa(lb/100 ft²) (11) (5) (53) (23)(25) (34) (17) (0) (56) Gels, Pa 7.7/21.5 4.8/14.8 24.9/45.4 12/25.33.8/5.7 18.2/36.4 8.6/21.5 4.3/14.8 13.9/28.7 23/35.4 (lb/100 ft²)(16/45) (10/31) (52/95) (25/53) (8/12) (38/76) (18/45) (9/31) (29/60)(48/74) API fluid 2.8 2.2 2.6 2.2 1.0 1.9 2.0 2.2 3.0 2.4 loss, cm³/30min 177° C. (350° F.) 52 — 31 16 — 21 16 16 — 15 HPHT (14 min) filtrate,cm³/30 min pH 8.8 8.6 8.7 8.6 9.4 8.5 8.4 8.4 6.6 9.2 Settling NoneAfter 16 hours of static aging at about 260° C. (500° F.), Formulas 21and 22 exhibited an HPHT filtrate of greater than 25 cm³/min after 30minutes or less at about 177° C. (350° F.), and Formulas 23 and 24exhibited an HPHT filtrate of less than 25 cm³/min after 30 minutes atabout 177° C. (350° F.). The result was believed to be attributable tovariability among commercially available polymers.

EXAMPLE 7

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Formulation 25 26 27 28 Experimental Water SolublePolymer A A A A Freshwater, 176 176 196 196 ml(bbl) (0.50) (0.50) (0.56)(0.56) MIL-GEL ™, 2.3 2.3 2.3 2.3 kg (lb) (5.0) (5.0) (4.0) (4.0)MIL-TEMP ®, 0.7 0.7 0.9 0.9 kg (lb) (1.5) (1.5) (2.0) (2.0) CHEM- 2.72.7 2.3 3.6 TROL ® X, kg (lb) (6.0) (6.0) (5.0) (8.0) Caustic soda, 0.230.23 0.23 0.34 kg (lb) (0.5) (0.5) (0.5) (0.75) KEM- 0.5 0.5 0.5 0.5SEAL ®  (1.0) (1.0) (1.0) (1.0) PLUS, kg (lb) Experimental 0.9 0.9 0.90.9 Water Soluble (2.0) (2.0) (2.0) (2.0) Polymer, kg (lb) MAX- — — — —TROL ®, kg (lb) POLYDRILL ™, 3.6 3.6 3.2 2.3 kg (lb) (8.0) (8.0) (7.0)(5.0) BOREDRILL ™, — — — — kg (lb) MIL-CARB ®, 4.5 4.5 4.5 4.5 kg (lb)(10) (10) (10) (10) MIL-BAR ®, 248.4 248.4 248.4 248.4 kg (lb) (547.7)(547.7) (547.7) (547.7) Caustic soda, — — — — kg (lb) Formulation 25 2627 28 FLUID PROPERTIES Fann 35 at 49° C. (120° F.) 260° C. (500° 260° C.260° C. 260° C. 260° C. 260° C. 260° C. 260° C. F.) (500° F.) (500° F.)(500° F.) (500° F.) (500° F.) (500° F.) (500° F.) Static Static StaticStatic Static Static Static Static Age 16 Age 16 Age 48 Age 72 Age 16Age 48 Age 72 Age 16 Initial Hours Initial Hours Hours Hours InitialHours Hours Hours Initial Hours 600 rpm >300 >300 >300 80 65 47 191 8592 79 202 92 300 rpm 242 249 238 46 32 25 100 50 41 41 106 56 200 rpm176 221 180 36 23 15 70 35 29 26 74 42 100 rpm 105 183 113 25 14 9 40 2116 14 41 27  6 rpm 20 124 26 12 5 2 6 4 3 2 6 9  3 rpm 16 122 22 12 4 14 4 2 1 4 9 Plastic NM NM NM 34 33 22 91 35 51 38 96 36 Viscosity, cPYield NM NM NM 5.7 −0.48 1.4 4.3 7.2 −4.8 1.4 4.8 9.6 Point, (12) (−1)(3) (9) (15) (−10) (3) (10) (20) Pa (lb/100 ft²) Gels, 9.6/18.2 61/92.914.4/ 10/22.5 3.8/11 2.4/5.3 2.9/4.3 5.3/13.9 2.9/11 1.4/5.7 2.9/3.88.1/20.6 Pa (20/38) (127/ 28.3 (21/47) (8/23) (5/11) (6/9) (11/29)(6/23) (3/12) (6/8) (17/43) (lb/100 ft²) 194) (30/59) API fluid 1.5 2.31.7 1.6 2.6 2.6 1.8 2.8 2.5 2.2 1.8 2.4 loss, cm³/30 min 177° C. — — —14 15 20 — 20 23 16 — 16 (350° F.) HPHT filtrate, cm³/30 min pH 8.5 9.06.9 9.0 9.8 9.1 9.2 9.0 8.9 8.9 9.8 8.6 Settling NoneAfter 16 hours of static aging at about 260° C. (500° F.), all of theFormulas exhibited an HPHT filtrate of less than 25 cm³/min after 30minutes at about 177° C. (350° F.). The result was believed to beattributable, at least in part, to the selection of polymer havingdesirable properties. Although not measured in this Example, onedesirable property for the water soluble polymer is a 5% by weightBrookfield viscosity in freshwater of from about 50 cP to about 100 cP.

EXAMPLE 8

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (D) 50:37.5:12.5mole % acrylamide:AMPS:acrylate or (A) 10 mole % AMPS polymerized withacrylamide. The formulas and the results are given in the followingTable: Formulation 29 30 31 32 Experimental Water Soluble Polymer D D AA Freshwater, 196 196 196 196 ml(bbl) (0.56) (0.56) (0.56) (0.56)MIL-GEL ™, 2.3 1.1 2.3 2.3 kg(lb) (4.0) (2.5) (4.0) (4.0) MIL-TEMP ®,1.1 1.1 0.9 0.9 kg(lb) (2.5) (2.5) (2.0) (2.0) CHEM-TROL ® 2.3 2.3 2.32.3 X, kg(lb) (5.0) (5.0) (5.0) (5.0) Caustic soda, 0.23 0.23 0.23 0.23kg(lb) (0.5) (0.5) (0.5) (0.5) KEM- 0.23 0.23 0.5 0.5 SEAL ® PLUS, (0.5)(0.5) (1.0) (1.0) kg(lb) Experimental 0.9 0.9 0.9 0.9 Water Soluble(2.0) (2.0) (2.0) (2.0) Polymer, kg(lb) POLYDRILL ™, 3.2 3.2 3.2 3.2kg(lb) (7.0) (7.0) (7.0) (7.0) MIL-CARB ®, 4.5 4.5 4.5 4.5 kg(lb) (10)(10) (10) (10) MIL-BAR ®, 248.4 248.4 248.4 248.4 kg(lb) (547.7) (547.7)(547.7) (547.7) Formulation 29 30 31 32 FLUID PROPERTIES Fann 35 at 49°C. (120° F.) 260° C. 260° C. 260° C. 260° C. (500° F.) (500° F.) (500°F.) (500° F.) Static Static Static Static Age 16 Age 16 Age 16 Age 16Initial Hours Initial Hours Initial Hours Initial Hours 600 rpm 117 89104 81 235 100 205 103 300 rpm 62 58 56 50 132 55 115 58 200 rpm 45 4639 39 94 41 81 44 100 rpm 25 34 22 26 54 24 46 29  6 rpm 4 18 3 13 8 5 710  3 rpm 3 16 2 11 6 4 5 8 Plastic Viscosity, 55 31 48 31 103 45 90 45Cp Yield Point, 3.4 12.9 3.8 9.1 13.9 4.8 12 6.2 Pa (7) (27) (8) (19)(29) (10) (25) (13) (lb/100 ft²) Gels, 2.4/3.8 12.4/26.8 1.4/2.97.7/16.3 3.4/4.8 3.8/13 3.8/5.7 7.7/27.3 Pa (5/8) (26/56) (3/6) (16/34)(7/10) (8/27) (8/12) (16/57) (lb/100 ft²) API fluid loss, 1.5 2.8 2.53.0 1.2 2.2 1.8 2.2 cm³/30 min 177° C. (350° F.) — 16 — 17 — 23 — 24HPHT filtrate, cm³/30 min pH 9.8 8.9 9.8 9.0 9.9 8.9 9.4 8.9 SettlingAfter 16 hours of static aging at about 260° C. (500° F.), all of theFormulas exhibited an HPHT filtrate of less than 25 cm³/min after 30minutes at about 177° C. (350° F.). The result was believed to beattributable, at least in part, to the selection of polymer havingdesirable properties.

EXAMPLE 9

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide; (C) 40 mole % AMPS polymerized withacrylamide; or (F) 90 weight percent NaAMPS (AMPS, sodium salt)/10weight percent N,N-dimethylacrylamide. The formulas and the results aregiven in the following Table: Formulation 33 34 35 36 37 ExperimentalWater Soluble Polymer A C F F A Freshwater, 196 196 196 196 196 ml(bbl)(0.56) (0.56) (0.56) (0.56) (0.56) MIL-GEL ™, 2.3 2.3 2.3 2.3 2.3 kg(lb) (4.0) (4.0) (4.0) (4.0) (4.0) MIL-TEMP ®, 0.9 0.9 0.9 0.9 †0.9 kg(lb) (2.0) (2.0) (2.0) (2.0) (2.0) (ALL-TEMP) CHEM-TROL ® 2.3 2.3 2.32.3 2.3 X, kg (lb) (5.0) (5.0) (5.0) (5.0) (5.0) Caustic soda, 0.23 0.230.23 0.23 0.23 kg (lb) (0.5) (0.5) (0.5) (0.5) (0.5) KEM- 0.5 0.5 — —0.5 SEAL ® PLUS, (1.0) (1.0) (1.0) lb Experimental 0.9 0.9 — — 0.9 WaterSoluble (2.0) (2.0) (2.0) Polymer, kg (lb) KEM-SEAL ® — — 1.4 1.4 — PLUS(3.0) (3.0) Experimental, kg (lb) POLYDRILL ™, 3.2 3.2 3.2 3.2 3.2 kg(lb) (7.0) (7.0) (7.0) (7.0) (7.0) MIL-CARB ®, 4.5 4.5 4.5 4.5 4.5(10)kg (lb) (10) (10) (10) (10) MIL-BAR ®, 248.4 248.4 248.4 248.4 248.4 kg(lb) (547.7) (547.7) (547.7) (547.7) (547.7) Formulation 33 34 35 36 37FLUID PROPERTIES Fann 35 at 49° C. (120° F.) 260° C. 260° C. 260° C.260° C. 260° C. (500° F.) (500° F.) (500° F.) (500° F.) (500° F.) StaticStatic Static Static Static Age 16 Age 16 Age 16 Age 16 Age 16 InitialHours Initial Hours Initial Hours Initial Hours Initial Hours 600 rpm166 91 153 96 230 218 197 235 254 118 300 rpm 93 51 85 61 131 176 111191 144 72 200 rpm 67 37 61 48 94 161 81 176 103 56 100 rpm 38 24 35 3454 138 46 151 59 37  6 rpm 7 6 6 11 9 104 7 115 9 10  3 rpm 5 4 4 9 6103 5 110 6 8 Plastic 73 40 68 35 99 42 86 44 110 46 Viscosity, cP YieldPoint, 9.6 5.3 8.1 12.4 14.8 64.2 12 70.4 16.3 12.4 Pa (20) (11) (17)(26) (32) (134) (25) (147) (34) (26) (lb/100 ft²) Gels, 3.4/5 5.2/18.22.9/4.3 10/28.2 3.8/5.7 55/76 3.4/4.8 60/84 3.8/5.7 9.6/29 Pa (7/10)(11/38) (6/9) (21/59) (8/12) (115/159) (7/10) (125/176) (8/12) (20/62)(lb/100 ft²) API fluid loss, 1.8 2.0 1.5 2.5 1.4 2.6 1.4 3.2 1.0 2.1cm³/30 min 149° C. (300° F.) — 44 — 49 — 54 — 52 — 26 HPHT filtrate, (13min) cm³/30 min pH 9.5 8.8 9.5 8.8 9.2 9.0 9.4 9.1 9.5 8.9*20% mole weight AMPS, **40% mole weight AMPS*** & ****lower molecular weight version of KEM-SEAL PLUS†substituted MIL-TEMP with ALL-TEMP to check compatibilityAfter 16 hours of static aging at about 260° C. (500° F.), all of theFormulas for which a measurement was taken exhibited an HPHT filtrate ofgreater than 25 cm³/min after 30 minutes or less at about 149° C. (300°F.). Formula 37, the 10 mole % AMPS polymerized with acrylamide (A)exhibited a HPHT filtrate of 26 cm³/min after 30 minutes or less atabout 149° C. (300° F.). The result was believed to be attributable tovariability among commercially available polymers and a lack ofsynergistic benefits of both experimental polymers and KEMSEAL® PLUS.

EXAMPLE 10

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Form. 40 41 42 43 44 45 Experiimental Water SolublePolymer A A A A A A Fresh- 196 196 196 196 196 196 water, (0.56) (0.56)(0.56) (0.56) (0.56) (0.56) ml(bbl) MIL- 2.3 2.3 2.3 2.3 2.3 2.3 GEL ™,(4.0) (4.0) (4.0) (4.0) (4.0) (4.0) kg (lb) MIL- 0.9 0.9 0.9 0.9 0.9 0.9TEMP ®, kg (2.0) (2.0) (2.0) (2.0) (2.0) (2.0) (lb) CHEM- 2.3 2.3 2.32.3 2.3 2.3 TROL ® X, (5.0) (5.0) (5.0) (5.0) (5.0) (5.0) kg (lb)Caustic 0.23 0.23 0.23 0.23 0.23 0.23 soda, kg (lb) (0.5) (0.5) (0.5)(0.5) (0.5) (0.5) KEM- 0.5 0.5 0.5 0.5 0.5 0.5 SEAL ®® (1.0) (1.0) (1.0)(1.0) (1.0) (1.0) PLUS, kg (lb) Experiimental 0.9 0.9 0.9 0.9 0.9 0.9Water (2.0) (2.0) (2.0) (2.0) (2.0) (2.0) Soluble Polymer, kg (lb) POLY-3.2 3.2 3.2 3.2 3.2 3.2 DRILL ™, (7.0) (7.0) (7.0) (7.0) (7.0) (7.0) kg(lb) MIL- 4.5 4.5 4.5 4.5 4.5 4.5 CARB ®, (10) (10) (10) (10) (10) (10)kg (lb) MIL- 248.4 248.4 248.4 248.4 248.4 248.4 BAR ®, (547.7) (547.7)(547.7) (547.7) (547.7) (547.7) kg (lb) Add 10% — 15.8 15.8 15.8 15.815.8 NaCl, 1(bbl) (0.1) (0.1) (0.1) (0.1) (0.1) MAX- — — 2.3 4.5 — —TROL ®, (5.0) (10) kg (lb) SOLTEX ®, — — — — 10 — kg (lb) SULFA- — — — —— 10 TROL ®, kg)lb) Form. 40 41 42 43 44 45 FLUID PROPERTIES Fann 35 at49° C. (120° F.) 260° C. 260° C. 260° C. 260° C. 260° C. 260° C. (500°F.) (500° F.) (500° F.) (500° F.) (500° F.) (500° F.) Static StaticStatic Static Static Static Age 16 Age 16 Age 16 Age 16 Age 16 Age 16Initial Hours Initial Hours Initial Hours Initial Hours Initial HoursInitial Hours 600 rpm 235 100 134 70 156 83 165 73 75 300 rpm 132 55 7442 87 54 113 46 53 200 rpm 94 41 52 34 62 45 92 36 42 100 rpm 54 24 3026 36 32 65 28 33  6 rpm 8 5 4 13 4 15 22 18 21  3 rpm 6 4 3 11 2 12 2116 19 Plastic 103 45 60 28 69 29 52 27 22 Viscosity, cP Yield Point,13.4 4.8 6.7 6.7 8.6 12 29.2 0.1 14.8 Pa (29) (10) (14) (14) (18) (25)(61) (19) (31) (lb/100 ft²) Gels, 3.4/4.8 3.8/13 2.4/4.3 10.5/27.32.4/6.7 10/23 14.4/26.8 11/23 13.9/27.3 Pa (7/10) (8/27) (5/9) (22/57)(5/14) (21/48) (30/56) (23/48) (29/57) (lb/100 ft²) API fluid 1.2 2.21.2 3.0 0.9 2.8 7.8 2.4 3.0 loss, cm³/30 min 177° C. (350° F.) — 23 — NC— >100 NC NC 48 HPHT filtrate, cm³/30 min Ph 9.9 8.9 9.1 8.5 9.0 8.5 8.58.5 8.5 SettlingAfter 16 hours of static aging at about 260° C. (500° F.), Formula 40exhibited an HPHT filtrate of less than 25 cm³/min after 30 minutes atabout 177° C. (350° F.). The remaining Formulas that were testedexhibited an HPHT filtrate of greater than 25 cm³/min after 30 minutesor less at about 177° C. (350° F.). The result was believed to beattributable to deleterious effects of salt to the base fluid. OnlyFormula 45, which included 10 lb/bbl SULFA-TROL®, exhibited restorationof HPHT filtration control.

EXAMPLE 12

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Form. 50 51 46 47 48 49 (repeat #40)] [(re-mix) #50] Experiimental Water Soluble Polymer A A A A A A Fresh- 196 196 196196 196 196 water, ml(bbl) (0.56) (0.56) (0.56) (0.56) (0.56) (0.56)MIL- 2.3 2.3 2.3 2.3 2.3 2.3 GEL ™, (4.0) (4.0) (4.0) (4.0) (4.0) (4.0)kg (lb) MIL- 0.9 0.9 0.9 0.9 0.9 0.9 TEMP ®, (2.0) (2.0) (2.0) (2.0)(2.0) (2.0) kg (lb) CHEM- 2.3 2.3 2.3 2.3 2.3 2.3 TROL ® X, (5.0) (5.0)(5.0 (5.0) (5.0) (5.0) kg (lb) Caustic 0.23 0.23 0.23 0.23 0.23 0.23soda, kg (lb) (0.5) (0.5) (0.5) (0.5) (0.5) (0.5) KEM- 0.5 0.5 0.5 0.50.5 0.5 SEAL ® (1.0) (1.0) (1.0) (1.0) (1.0) (1.0) PLUS, kg (lb)Experiimental 0.9 0.9 0.9 0.9 0.9 0.9 Water (2.0) (2.0) (2.0) (2.0)(2.0) (2.0) Soluble Polymer, kg (lb) POLY- 3.2 3.2 3.2 3.2 3.2 3.2DRILL ™, (7.0) (7.0) (7.0) (7.0) (7.0) (7.0) kg (lb) MIL- 4.5 4.5 4.54.5 4.5 4.5 CARB ®, (10) (10) (10) (10) (10) (10) kg (lb) MIL- 248.4248.4 248.4 248.4 248.4 248.4 BAR ®, kg (lb) (547.7) (547.7) (547.7)(547.7) (547.7) (547.7) Add 10% 15.8 15.8 15.8 15.8 — — NaCl, 1(bbl)(0.1) (0.1) (0.1) (0.1) SULFA- — 2.3 4.5 — — — TROL ®, (5.0) (10) kg(lb) PROTECTO- 4.5 2.3 — — — — MAGIC ™, (10) (5.0) kg (lb) CHEK- — — 4.5— — — LOSS ® (10) PLUS, kg)lb) Bore-Drill, — — — 4.5 — — kg (lb) (10)Form. 50 51 FLUID PROPERTIES Fann 35 at 49° C. (120° F.) 260° C. 260° C.260° C. 260° C. 260° C. 260° C. (500° F.) (500° F.) (500° F.) (500° F.)(500° F.) (500° F.) Static Static Static Static Static Static Age 16 Age16 Age 16 Age 16 Age 16 Age 16 Initial Hours Initial Hours Initial HoursInitial Hours Initial Hours Initial Hours 600 rpm 126 81 111 Cannot 260100 247 73 300 rpm 93 57 77 Stir 140 52 137 39 200 rpm 81 48 70 Down 10536 98 26 100 rpm 66 37 58 58 20 56 14  6 rpm 49 25 40 8 3 8 2  3 rpm 4723 39 6 2 6 1 Plastic 33 24 34 120 48 110 34 Viscosity, cP Yield Point,28.7 11 20.6 9.6 1.9 12.9 2.4 Pa (60) (23) (43) (20) (4) (27) (5)(lb/100 ft²) Gels, 24.9/36.4 15.8/33.5 27.3/52.7 3.8/4.8 1.4/11 3.4/5.2.96/6.7 Pa (51/76) (33/70) (57/110) (8/10) (3/23) (7/11) (2/14) (lb/100ft²) API fluid 2.8 2.8 2.2 1.0 1.7 1.0 2.0 loss, cm³/30 min 177° C.(350° F.) 62 80 80 — 44 — 46 HPHT filtrate, cm³/30 min pH 8.3 8.4 8.39.5 8.9 9.1 8.8After 16 hours of static aging at about 260° C. (500° F.), all of theFormulas tested exhibited an HPHT filtrate of greater than 25 cm³/minafter 30 minutes at about 177° C. (350° F.). The result was believed tobe attributable to the deleterious effects of salt and variability amongcommercially available polymers. Testing with other commerciallyavailable additives did not restore HPHT filtration control.

EXAMPLE 13

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Formulation 52 53 54 55 56 57 Experimental WaterSoluble Polymer A A A A A A Fresh- 196 196 196 196 196 196 water,ml(bbl) (0.56) (0.56) (0.56) (0.56) (0.56) (0.56) MIL- 2.3 2.3 2.3 2.32.3 2.3 GEL ™, (4.0) (4.0) (4.0) (4.0) (4.0) (4.0) kg(lb) MIL- 0.9 0.90.9 0.9 0.9 0.9 TEMP ®, (2.0) (2.0) (2.0) (2.0) (2.0) (2.0) kg(lb) CHEM-2.3 2.3 2.3 2.3 2.3 2.3 TROL ® X, (5.0) (5.0) (5.0) (5.0) (5.0) (5.0)kg(lb) Caustic 0.23 0.23 0.23 0.23 0.23 0.23 soda, kg(lb) (0.5) (0.5)(0.5) (0.5) (0.5) (0.5) KEM- 0.5 0.5 0.5 0.5 0.5 0.5 SEAL ® (1.0) (1.0)(1.0) (1.0) (1.0) (1.0) PLUS, kg(lb) Experimental 0.9 0.9 0.9 0.9 0.90.9 Water (2.0) (2.0) (2.0) (2.0) (2.0) (2.0) Soluble Polymer, kg(lb)POLY- 3.2 3.2 3.2 3.2 3.2 3.2 DRILL ™, (7.0) (7.0) (7.0) (7.0) (7.0)(7.0) kg(lb) MIL- 4.5 4.5 4.5 4.5 4.5 4.5 CARB ®, (10) (10) (10) (10)(10) (10) kg(lb) MIL- 248.4 248.4 248.4 248.4 248.4 248.4 BAR ®, kg(lb)(547.7) (547.7) (547.7) (547.7) (547.7) 547.7 Add 10% 15.8 15.8 — — — —NaCl, l(bbl) (0.1) (0.1) SULFA- — 5 — — — — TROL ®, kg(lb) BORE- 2.3 2.3— — — — DRILL ™, (5) (5) kg(lb) Form. 52 53 54 55 56 57 FLUID PROPERTIESFann 35 at 49° C.(120° F.) 260° C. 260° C. 260° C. 260° C. 260° C. 260°C. (500° F.) (500° F.) (500° F.) (500° F.) (500° F.) (500° F.) StaticStatic Static Static Static Static Age 16 Age 16 Age 16 Age 16 Age 16Age 16 Initial Hours Initial Hours Initial Hours Initial Hours InitialHours Initial Hours 600 rpm 197 227 274 93 192 125 189 110 174 103 300rpm 140 160 153 52 103 65 104 58 95 57 200 rpm 116 136 110 38 72 47 7542 67 41 100 rpm 87 100 62 22 41 28 42 24 38 24 6 rpm 40 43 8 4 7 9 7 56 5 3 rpm 35 40 6 3 5 7 5 3 4 3 Plastic 37 67 121 41 89 60 85 52 79 46Viscosity, Cp Yield Point, 49.3 44.5 15.3 5.3 6.7 2.4 9.1 2.9 7.7 5.3 Pa(103) (93) (32) (11) (14) (5) (19) (6) (16) (11) (lb/100 ft²) Gels,23/43.6 24.449.8 3.4/5.7 3.4/13.4 2.9/5.3 8.1/25.9 2.9/4.8 3.8/15.8 2.9/4.9/20.1 Pa (48/91) (51/104) (7/12) (7/28) (6/11) (17/54) (6/10) (8/33)4.3 (10/42) (lb/100 ft²) (6/9) API fluid 3.8 5.4 1.4 2.0 1.2 2.4 1.0 2.21.2 2.2 loss, cm³/ 30 min 177° C. — — — 50 — 38 — 28 — 40 (350° F.) HPHTfiltrate, cm³/30 min pH 8.2 8.5 9.6 8.7 9.7 8.7 9.7 8.7 9.8 8.8 SettlingAfter 16 hours of static aging at about 260° C. (500° F.), all of theFormulas tested exhibited an HPHT filtrate of greater than 25 cm³/minafter 30 minutes at about 177° C. (350° F.). The result was believed tobe attributable, at least in part, to variability among commerciallyavailable polymers.

EXAMPLE 14

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Formulation 58 59 60 Experimental Water SolublePolymer A A A Freshwater, 196 196 196 ml(bbl) (0.56) (0.56) (0.56)MIL-GEL ™, 2.3 2.3 2.3 kg(lb) (4.0) (4.0) (4.0) MIL-TEMP ®, 0.9 0.9 0.9kg(lb) (2.0) (2.0) (2.0) CHEM-TROL ® 2.3 2.3 2.3 X, kg(lb) (5.0) (5.0)(5.0) Caustic soda, 0.23 0.23 0.23 kg(lb) (0.5) (0.5) (0.5) KEM- 0.5 0.50.5 SEAL ®PLUS, (1.0) (1.0) (1.0) kg(lb) Experimental 2 2 2 WaterSoluble Polymer, kg(lb) POLYDRILL ™, 3.2 3.2 3.2 kg(lb) (7.0) (7.0)(7.0) MIL-CARB ®, 4.5 4.5 4.5 kg(lb) (10) (10) (10) MIL-BAR ®, 248.4248.4 248.4 kg(lb) (547.7) (547.7) (547.7) 10% NaCl, l(bbl) — 15.8 15.8(0.1) (0.1) SULFA- — 4.5 TROL ®, kg(lb, (10) POLYDRILL ™, — — 4.5 kg(lb)(10) Formulation 58 59 60 FLUID PROPERTIES Fann 35 at 49° C.(120° F.)260° C. 260° C. 260° C. 260° C. (500° F.) (500° F.) (500° F.) (500° F.)Static Static Static Static Age 16 Age 72 Age 16 Age 16 Initial HoursHours Hours Hours 600 rpm 216 90 77 104 113 300 rpm 120 51 45 76 84 200rpm 86 39 32 65 70 100 rpm 48 24 19 52 58 6 rpm 8 5 4 30 37 3 rpm 5 4 329 35 Plastic 96 39 32 28 29 Viscosity, Cp Yield Point, 11.5 5.7 6.2 2326.3 Pa(lb/100 ft²) (24) (12) (13) (48) (55) Gels, 3.8/ 4.8/20.12.4/13.4 19.6/38.3 20.6/35.4 Pa 10.5 (10/42) (5/28) (41/80) (43/74)(lb/100 ft²) (8/ 22) API fluid loss, 1.6 2.4 2.4 2.4 2.0 cm³/30 min 177°C.(350° F.) — 34 20 24 NC HPHT filtrate, cm³/30 min pH 9.6 8.7 8.5 8.38.3After 16 hours of static aging at about 260° C. (500° F.), Formulas 58(no added NaCl) and 59 (10 lb/bbl SULFA-TROL® and added NaCl) exhibitedan HPHT filtrate of less than 25 cm³/min after 30 minutes at about 177°C. (350° F.). Formula 60, which contained added NaCl, but notSULFA-TROL®, exhibited an HPHT filtrate of greater than 25 cm³/min after30 minutes at about 177° C. (350° F.).

EXAMPLE 15

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Formulation 61 62 63 64 65 66 Experimental WaterSoluble Polymer **16-hr aged A sample A A A A Fresh- 196 196 196 196 196water, (0.56) (0.56) (0.56) (0.56) (0.56) ml(bbl) MIL-GEL ™, 2.3 2.3 2.32.3 2.3 kg(lb) (4.0) (4.0) (4.0) (4.0) (4.0) MIL-TEMP ®, 0.9 0.9 0.9 0.90.9 kg(lb) (2.0) (2.0) (2.0) (2.0) (2..0) CHEM-TROL ® X, 2.3 2.3 2.3 2.32.3 kg(lb) (5.0) (5.0) (5.0) (5.0) (5.0) Caustic soda, 0.23 0.23 0.230.23 0.23 kg(lb) (0.5) (0.5) (0.5) (0.5) (0.5) KEM-SEAL ®® 0.5 0.5 0.50.5 0.5 PLUS, kg(lb) (1.0) (1.0) (1.0) (1.0) (1.0) Experimental 0.9 0.90.9 0.9 0.9 Water Soluble (2) (2) (2) (2) (2) Polymer, kg(lb)POLYDRILL ™, 3.2 3.2 3.2 3.2 3.2 kg(lb) (7.0) (7.0) (7.0) (7.0) (7.0)MIL-CARB ®, 4.5 4.5 4.5 4.5 4.5 kg(lb) (10) (10) (10) (10) (10) MIL-BAR,248.4 248.4 248.4 248.4 248.4 kg(lb) (547.7) (547.7) (547.7) (547.7)(547.7) 10% NaCl ®, — 15.8 15.8 15.8 15.8 15.8 l(bbl) (0.1) (0.1) (0.1)(0.1) (0.1) SULFA-TROL ®, — 4.5 4.5 2.3 2.3 kg(lb) (10) (10) (5.0) (5.0)POLYDRILL ™, — — 4.5 — kg(lb) (10) PROTECTO- — — — — 2.3 MAGIC, (5.0)kg(lb) Caustic soda, kg(lb — 0.23 — — — — (0.5) Form. 61 62 63 64 65 66FLUID PROPERTIES Fann 35 at 149° C. (120° F.) 260° C. 260° C. 260° C.260° C. 260° C. (500° F.) (500° F.) (500° F.) 260° C. 260° C. (500° F.)(500° F.) Static Static Static (500° F.) (500° F.) Static Static Age 16Age 72 Age 16 Static Age Static Age Age 16 Age 16 Initial Hours HoursHours 16 Hours 16 Hours Hours Hours 600 rpm 216 90 77 133 104 113 116143 300 rpm 120 51 45 103 76 84 85 110 200 rpm 86 39 32 92 65 70 75 98100 rpm 48 24 19 77 52 58 60 84 6 rpm 8 5 4 55 30 37 40 60 3 rpm 5 4 352 29 35 37 57 Plastic Viscosity, 96 39 32 30 28 29 31 33 cP YieldPoint, 11.5 5.7 6.2 35 23 26.3 25.9 36.9 Pa (24) (12) (13) (73) (48)(55) (54) (77) (lb/100 ft²) Gels, Pa 3.8/10.5 4.8/20.1 2.4/13.429.6/49.3 19.6/38.3 20.6/35.4 24/49.8 32.6/55 (lb/100 ft²) (8/22)(10/42) (5/28) (62/103) (41/80) (43/74) (50/104) (68/115) API fluidloss, 1.6 2.4 2.4 2.0 2.4 2.0 2.8 3.0 cm³/30 min 177° C.(350° F.) — 3420 22 24 NC >100 86 HPHT filtrate, cm³/30 min Ph 9.6 8.7 8.5 8.5 8.3 8.38.4 8.3**16-hr aged sample was sample 61, which was already aged at 500° F. for16 hours, and then was contaminated with 10% by vol. 10% NaCl brine,followed with 10 lb/bbl SULFA-TROL ® and 0.23(0.5) lb/bbl caustic sodaAfter 16 hours of static aging at about 260° C. (500° F.), Formula 63and reaged Formula 62, which contained both NaCl and SULFA-TROL®,exhibited an HPHT filtrate of less than 25 cm³/min after 30 minutes atabout 177° C. (350° F.). The remainder of the formulas exhibited an HPHTfiltrate of greater than 25 cm³/min after 30 minutes or less at about177° C. (350° F.).

EXAMPLE 16

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. The formulas and the results are given inthe following Table: Form. 67 68 69 70 71 72 Experimental Water SolublePolymer A A A A A A Fresh- 196 196 196 196 196 196 water, (0.56) (0.56)(0.56) (0.56) (0.56) (0.56) ml(bbl) MIL-GEL ™, 2.3 2.3 2.3 2.3 2.3 2.3kg(lb) (4.0) (4.0) (4.0) (4.0) (4.0) (4.0) MIL- 0.9 0.9 0.9 0.9 0.9 0.9TEMP ®, (2.0) (2.0) (2.0) (2.0) (2.0) (2.0) kg(lb) CHEM- 2.3 2.3 2.3 2.32.3 2.3 TROL ® X, (5.0) (5.0) (5.0) (5.0) (5.0) (5.0) kg(lb) Caustic0.23 0.23 0.23 0.23 0.23 0.23 soda, kg(lb) (0.5) (0.5) (0.5) (0.5) (0.5)(0.5) KEM- 0.5 0.5 0.5 0.5 0.5 0.5 SEAL ® (1.0) (1.0) (1.0) (1.0) (1.0)(1.0) PLUS, kg(lb) Experimental 0.9 0.9 0.9 0.9 0.9 2.3 Water (2) (2)(2) (2) (2.0) (4.0) Soluble Polymer, kg(lb) POLY- 3.2 3.2 3.2 3.2 3.23.2 DRILL ™, (7.0) (7.0) (7.0) (7.0) (7.0) (7.0) (kg/lb) MIL- 4.5 4.54.5 4.5 4.5 4.5 CARB ®, kg(lb) (10) (10) (10) (10) (10) (10) MIL-BAR ®,248.4 248.4 248.4 248.4 248.4 248.4 kg(lb) (547.7) (547.7) (547.7)(547.7) (547.7) (547.7) 10% — 15.8 15.8 15.8 — — NaCl, l(bbl) (0.1)(0.1) (0.1) SULFA- — 4.5 — — — — TROL ®, (10) kg(lb) MAX- — — 4.5 — — —TROL ®, (10) kg(lb) SOLTEX ®, — — 4.5 — — kg(lb) (10) Form. 67 68 69 7071 72 FLUID PROPERTIES Fann 35 at 49° C.(120° F.) 260° C. 260° C. 260°C. 260° C. 260° C. 260° C. (500° F.) (500° F.) (500° F.) (500° F.) (500°F.) (500° F.) Static Static Static Static Static Static Age 16 Age 16Age 16 Age 16 Age 16 Age 16 Initial Hours Initial Hours Initial HoursInitial Hours Initial Hours Initial Hours 600 rpm 182 102 220 101 >30077 264 70 300 rpm 99 72 159 75 >300 36 145 40 200 rpm 70 60 134 64 22422 103 31 100 rpm 39 48 103 52 131 12 58 19 6 rpm 6 30 48 33 21 2 9 7 3rpm 4 27 42 31 17 1 6 6 Plastic 83 30 61 26 NM 41 119 30 Viscosity, CpYield Point, 7.7 20.1 46.9 23.5 NM 0 12.4 4.8 Pa (16) (42) (98) (49) (0)(26) (10) (lb/100 ft²) Gels, 2.9/ 17.7/29.7 27.3/43.6 17.7/26.3 9.6/16.31.9/4.3 3.8/8.6 5.3/11.5 Pa 3.8 (37/62) (57/91) (37/55) (20/34) (4/9)(8/18) (11/24) (lb/100 ft²) (6/8) API fluid 1.5 2.3 4.4 2.7 2.0 2.0 1.82.6 loss, cm³/30 min 177° C.(350° F.) — 21 60 >100 — 16 — 18 HHPTfiltrate, cm³/30 min pH 9.8 8.5 9.0 8.6 7.9 8.6 8.1 8.5

After 16 hours of static aging at about 260° C. (500° F.), the control(no added NaCl, Formulas 71 and 72) and Formula 68 (containing 10 lb.bblSULFA-TROL®) exhibited an HPHT filtrate of less than 25 cm³/min after 30minutes at about 177° C. (350° F.). The remainder of the formulasexhibited an HPHT filtrate of greater than 25 cm³/min after 30 minutesor less at about 177° C. (350® F.). This was true even though Formula 70contained SOLTEX®, a sulfonated asphalt product. The result was believedto be attributable to the fact that SULFA-TROL® has a % lignite content.Lignite has limited solubility in saline water, which may account forimproved HPHT filtration control compared with SOLTEX®. It is believedthat the insoluble portion of the colloidal lignite will impart particleplugging onto the filter cake, thereby slowing down filtration. SOLTEX®apparently is too soluble to impart particle plugging.

In order to determine the differences between the two sulfonatedasphalts, a measurement was made of “High pH Soluble but Low pHInsoluble” Materials for SOLTEX® and SULFA-TROL® Samples, as compared toLIGCO® (pure lignite) Samples LIGCO ® SOLTEX ® SULFA-TROL ® % soluble atpH = 11 but 77% 0.5% (test 1) 28% (test 1) insoluble at pH = 2.5 0.8%(test 2) 25% (test 2) % “Lignite” content* 100%   1% 34%*Note: Calculated based on that lignite is 77% soluble at pH = 11.

EXAMPLE 17

In the following test, the drilling fluid system had the followinggeneral formula, where the water soluble polymer was (A) 10 mole % AMPSpolymerized with acrylamide. Each of the formulations contained 10lb/bbl SULFA-TROL®, with varying levels of NaCl. The formulas and theresults are given in the following Table: Formulation 73 74 75 76Experimental Water Soluble Polymer A A A A Freshwater, ml(bbl) 196 196196 196 (0.56) (0.56) (0.56) (0.56) MIL-GEL ™, kg(lb) 2.3 2.3 2.3 2.3(4.0) (4.0) (4.0) (4.0) NaCI, kg(lb) 1.8 1.8 3.6 3.6 (3.9) (3.9) (8.0)(8.0) MIL-TEMP ®, kg(lb 0.9 0.9 0.9 0.9 (2.0) (2.0) (2.0) (2.0)CHEM-TROL ® X, kg(lb) 2.3 2.3 2.3 2.3 (5.0) (5.0) (5.0) (5.0) Causticsoda, kg(lb) 0.23 0.23 0.23 0.23 (0.5) (0.5) (0.5) (0.5) KEM-SEAL ®PLUS, 0.5 0.5 0.5 0.5 kg(lb) (1.0) (1.0) (1.0) (1.0) Experimental Water0.9 — 0.9 — Soluble Polymer, kg(lb) (2.0) (2.0) POLYDRILL ™, kg(lb) 3.23.2 3.2 3.2 (7.0) (7.0) (7.0) (7.0) MIL-CARB ®, kg(lb) 4.5 4.5 4.5 4.5(10) (10) (10) (10) MIL-BAR ®, kg(lb) 248.4 248.4 248.4 248.4 (547.7)(547.7) (547.7) (547.7) SULFA-TROL ®, kg(lb) 4.5 4.5 4.5 4.5 (10) (10)(10) (10) Formulation 73 74 75 76 FLUID PROPERTIES Fann 35 at 49° C.(120° F.) 260° C. 260° C. 260° C. (500° F.) (500° F.) (500° F.) 260° C.Static Static Static (500° F.) Age 16 Age 16 Age 16 Static Age InitialHours Initial Hours Initial Hours Initial 16 Hours 600 rpm 190 159 109181 172 127 107 105 300 rpm 104 113 62 142 95 87 60 80 200 rpm 75 95 44128 67 71 43 71 100 rpm 42 74 26 109 38 55 26 59  6 rpm 6 45 4 85 6 34 547  3 rpm 4 42 3 82 3 31 4 44 Plastic Viscosity, cP 86 46 47 39 77 40 4725 Yield Point, 8.6 32 7.2 49.3 8.6 22.5 6.2 26.3 Pa (18) (67) (15)(103) (18) (47) (13) (55) (lb/100 ft²) Gels, Pa 3.4/8.6 25.4/39.33.4/6.2 3.4/47.4 3.4/10 18.7/29.6 3.8/8.6 20.6/33.5 (lb/100 ft²) (7/18)(53/82) (7/13) (7/99) (7/21) (39/62) (8/18) (43/70) API fluid loss,cm³/30 0.9 2.0 0.6 2.5 1.0 2.0 1.5 2.7 min 177° C. (350° F.) HPHT — 19 —NC — NC — NC filtrate, cm³/30 min Ph 9.1 8.6 9.3 8.9 8.9 8.2 9.0 8.6After 16 hours of static aging at about 260° C. (500° F.), the onlyformula tested that exhibited an HPHT filtrate of less than 25 cm³/minafter 30 minutes at about 177° C. (350° F.) was Formula 73, whichcontained 2 lb/bbl of the experimental water soluble polymer, 3.9 lb/bblNaCl (or about 9000 ppm NaCl) and 10 lb/bbl SULFA-TROL®. Examples 75 and76 contained 8.0 lb/bbl NaCl, or from about 18,000 to about 20,000 ppmNaCl, and Example 76 did not include the experimental water solublepolymer.

Persons of ordinary skill in the art will recognize that manymodifications may be made to the foregoing without departing from thespirit and scope thereof. The embodiment described herein is meant to beillustrative only and should not be taken as limiting the invention,which is defined in the following claims.

1. A method for providing filtration control during drilling operationsusing a drilling fluid system comprising water soluble polymercomprising acrylamide monomer, the method comprising: providing adrilling fluid system having effective rheological properties, thedrilling fluid system comprising aqueous base consisting essentially offreshwater, a quantity of from about 21 to 58 kg/m³ (7.5 to about 20pounds per barrel) plugging agent, and an amount of bridging agentcomprising water soluble polymer comprising acrylamide monomer whichexhibits a 5% by weight Brookfield viscosity in freshwater of from about50 cP to about 100 cP, and; contacting a subterranean formation with thedrilling fluid system and maintaining effective filtration control atconcentrations of about 10,000 ppm or less monovalent salt, asdemonstrated in the laboratory by producing a HPHT filtrate of 25cm³/min or less after 30 minutes at about 149° C. (300° F.) after staticaging of said drilling fluid system for a period of 16 hours or more attemperatures of about 260° C. (500° F.).
 2. The method of claim 1comprising providing the drilling fluid system with said water solublepolymer comprising one or more additional monomer selected from thegroup consisting of sulfonate monomer and a combination of sulfonatemonomer and N-vinyl lactam monomer.
 3. The method of claim 1 comprisingproviding the drilling fluid system with plugging agent comprisingsulfonated asphalt and lignite.
 4. The method of claim 1 furthercomprising exposing said drilling fluid system to a temperature of 177°C. (350° F.) for 16 hours or more, said drilling fluid systemmaintaining effective filtration control.
 5. The method of claim 1further comprising exposing the drilling fluid system to a temperatureof 260° C. (500° F.) for 16 hours or more, said drilling fluid systemmaintaining effective filtration control.
 6. The method of claim 2further comprising exposing the drilling fluid system to a temperatureof 260° C. (500° F.) for 16 hours or more, said drilling fluid systemmaintaining effective filtration control.
 7. The method of claim 3further comprising exposing the drilling fluid system to a temperatureof 260° C. (500° F.) for 16 hours or more, said drilling fluid systemmaintaining effective filtration control.
 8. A drilling fluid systemcomprising: an aqueous base consisting essentially of freshwater; aquantity of from about 21 to 58 kg/m³ (7.5 to 20 lb/bbl) plugging agentcomprising sulfonated asphalt and lignite; bridging agent comprising anamount of water soluble polymer having a 5% by weight Brookfieldviscosity in freshwater of from about 50 cP to about 100 cP, the watersoluble polymer comprising first olefinic amide monomer copolymerizedwith monomer selected from the group consisting of sulfonate monomer anda combination of sulfonate monomer and N-vinyl lactam monomer, wherein:the first olefinic amide monomer has the following structure:

wherein R is a 1-alkenyl group and R¹ and R² independently are selectedfrom the group consisting of hydrogen and alkyl groups having from 1 to4 carbon atoms; the sulfonate monomer has the following structure:

wherein R is selected from the group consisting of polymerizable alkenylgroups having from about 1 to about 4 carbon atoms; R¹ and R² areselected from the group consisting of hydrogen and methyl groups; and, Xis a cation; and, the acrylate monomer has the following generalstructure:

wherein R⁶ is selected from the group consisting of hydrogen, alkylgroups having from about 1 to about 4 carbon atoms, and groups thatactivate the unsaturated carbon-carbon bond; R⁷ and R⁸ independently areselected from the group consisting of hydrogen and alkyl groups havingfrom about 1 to about 4 carbon atoms; and, Y is selected from the groupconsisting of hydrogen and alkali metal; and, the N-vinyl lactam monomerhas the following structure:

wherein R⁹, R¹⁰, R¹¹, and R¹² independently are selected from the groupconsisting of hydrogen, methyl groups, and ethyl groups; wherein, after16 hours of static aging of the drilling fluid system at about 260° C.(500° F.), the drilling fluid system exhibits a HPHT filtrate of 25cm³/min or less after 30 minutes at about 149° C. (300° F.).
 9. Adrilling fluid system comprising: an aqueous base consisting essentiallyof freshwater; bridging agent comprising from about 0.7 to about 29kg/m³ (0.25 lb/bbl to about 10.0 lb/bbl) water soluble polymer having a5% by weight Brookfield viscosity in freshwater of from about 50 cP toabout 100 cP, the water soluble polymer comprising acrylamide monomercopolymerized with monomer selected from the group consisting ofsulfonate monomer and a combination of sulfonate monomer and N-vinyllactam monomer; and, a quantity of from about 21 to 58 kg/m³ (7.5 to 20lb/bbl) plugging agent comprising sulfonated asphalt and lignite;wherein, after 16 hours of static aging of the drilling fluid system atabout 260° C. (500° F.), the drilling fluid system exhibits a HPHTfiltrate of 25 cm³/min or less after 30 minutes at about 149° C. (300°F.).
 10. The drilling fluid system of claim 9 exhibiting an HPHTfiltrate of 25 cm³/min or less after 30 minutes at about 177° C. (350°F.).
 11. The drilling fluid system of claim 9 wherein the sulfonatemonomer comprises 2-acrylamido-2-methylpropane sulfonic acid or alkalimetal salts thereof.
 12. The drilling fluid system of claim 11exhibiting said HPHT filtrate at a content of 10,000 ppm or lessmonovalent salt.
 13. The drilling fluid system of claim 11 exhibitingsaid HPHT filtrate at a content of 9,000 ppm or less monovalent salt,wherein said amount of water soluble polymer is from about 2.8 to about11.5 kg/m³ (1 to about 4 lb/bbl).
 14. The drilling fluid system of claim9 wherein the plugging agent comprises from about 20 wt. % to about 30wt. % lignite.
 15. A drilling fluid system comprising: an aqueous baseconsisting essentially of freshwater; bridging agent comprising fromabout 0.7 to about 29 kg/m³ (0.25 lb/bbl to about 10.0 lb/bbl) watersoluble polymer having a 5% by weight Brookfield viscosity in freshwaterof from about 50 cP to about 100 cP, said water soluble polymercomprising monomer of acrylamide, monomer of2-acrylamido-2-methylpropane sulfonic acid or alkali metal saltsthereof, and N-vinyl pyrrolidone monomer; and, from about 21.42 to 57.15kg/m³ (7.5 to 20 lb/bbl) plugging agent comprising sulfonated asphaltand lignite; wherein, after 16 hours of static aging of the drillingfluid system at about 260° C. (500° F.), the drilling fluid systemexhibits a HPHT filtrate of 25 cm³/min or less after 30 minutes at about149° C. (300° F.).
 16. The drilling fluid system of claim 15 exhibitinga HPHT filtrate of 25 cm³/min or less after 30 minutes at about 177° C.(350° F.).
 17. The drilling fluid system of claim 15 exhibiting saidHPHT filtrate at a content of 10,000 ppm or less monovalent salt. 18.The drilling fluid system of claim 15 exhibiting said HPHT filtrate at acontent of 9,000 ppm or less monovalent salt, wherein said amount ofwater soluble polymer is from about 2.8 to about 11.5 kg/m³ (1 to about4 lb/bbl).
 19. The drilling fluid system of claim 15 wherein said watersoluble polymer comprises: from about 40 to about 60 mole % acrylamide;from about 10 to about 20 mole % 2-acrylamido-2-methylpropane sulfonicacid; and, from about 50 to about 20 mole % N-vinyl pyrrolidone.
 20. Thedrilling fluid system of claim 15 wherein said water soluble polymercomprises: about 55 mole % 2-acrylamido-2-methylpropane sulfonic acid;and, about 30 mole % N-vinyl pyrrolidone; and. about 15 mole %acrylamide.
 21. The drilling fluid system of claim 15 wherein theplugging agent comprises from about 20 wt. % to about 30 wt. % lignite.22. A drilling fluid system comprising: an aqueous base consistingessentially of freshwater; bridging agent comprising from about 0.7 toabout 29 kg/m³ (0.25 lb/bbl to about 10.0 lb/bbl) water soluble polymerhaving a 5% by weight Brookfield viscosity in freshwater of from about50 to about 100 cP comprising monomers of acrylamide and2-acrylamido-2-methylpropane sulfonic acid or alkali metal saltsthereof; and, plugging agent comprising a quantity of from about 21 to58 kg/m³ (7.5 to 20 lb/bbl) sulfonated asphalt comprising lignite;wherein, after 16 hours of static aging of the drilling fluid system atabout 260° C. (500° F.), the drilling fluid system exhibits a HPHTfiltrate of 25 cm³/min or less after 30 minutes at about 149° C. (300°F.).
 23. The drilling fluid system of claim 22 exhibiting a HPHTfiltrate of 25 cm³/min or less after 30 minutes at about 177° C. (350°F.).
 24. The drilling fluid system of claim 22 exhibiting said HPHTfiltrate at a content of 10,000 ppm or less monovalent salt.
 25. Thedrilling fluid system of claim 22 exhibiting said HPHT filtrate at acontent of 9,000 ppm or less monovalent salt, wherein said amount ofwater soluble polymer is from about 2.8 to about 11.5 kg/m³ (1 to about4 lb/bbl).
 26. The drilling fluid system of claim 22 wherein said watersoluble polymer comprises: from about 90 to about 80 mole % acrylamide;and, from about 10 to about 20 mole % 2-acrylamido-2-methylpropanesulfonic acid.
 27. The drilling fluid system of claim 22 wherein saidwater soluble polymer comprising about 55 mole %2-acrylamido-2-methylpropane sulfonic acid; and, about 15 mole %acrylamide.
 28. The drilling fluid system of claim 22 wherein theplugging agent comprises from about 20 wt. % to about 30 wt. % lignite.